The present invention relates to a torque sensor for detecting a torque generated at a rotation shaft, and more particularly to a torque sensor installed to an electric power steering apparatus.
Japanese Patent Provisional Publication No. (Heisei) 2-132336 discloses a torque sensor for detecting a torque generated at a shaft by detecting a twist quantity between first and second shafts connected through a torsion bar. More specifically, such a torque sensor is arranged to detect a change of a relative rotational position between a first induction member (covered member) on the first shaft and a second induction member (cover member) on the second shaft by detecting an impedance change of a coil set around the induction members.
However, since the conventional torque sensor is arranged to simply locate the cover member between a detection coil and the covered member, some magnetic field bypassing the cover member is generated. This bypassing magnetic field decreases a quantity of the impedance change of a coil according to the twist quantity and degrades the detection accuracy of detecting a torque generated at a shaft.
It is an object of the present invention to provide an improved torque sensor which ensures a high detection accuracy of torque detection by shutting out a magnetic field bypassing an outer periphery of a cover member.
A torque sensor according to the present invention comprises a first shaft, an elastic member, a second shaft connected to the first shaft through the elastic member, a covered member, a covered member, a detector coil and an outer cylinder portion. The covered member is fixed to the first shaft and is made of magnetic material. The covered member comprises a plurality of cutout portions formed at predetermined intervals in a circumferential direction of the covered member. The cutout portions are located on at least one of both covered-member side surfaces perpendicular to an axis of the first shaft. The cover member is fixed to the second shaft and faced with a plane including the cutout portions of the covered member. The cover member is made of conductive and non-magnetic material and comprises a plurality of cutout portions which are formed corresponding the cutout portions of the covered member. The detector coil detects a change of an overlapped quantity between the cutout portions of the covered member and the cutout portions of the cover member on the basis of a change of an impedance as a torque quantity generated between the first and second shafts. The outer cylinder portion is integrally connected to an outer periphery of the cover member, the outer cylinder portion extending from the outer periphery of the cover member along an outer circumference of the covered member.